It's not hard for the most part. Maybe the toughest thing is finding somebody you trust who can play like you do to plunk some chords and such while you do the listening.
If you can, have your stand-in play some things, nothing too busy, while you listen to which of the four speakers sounds the best to you. Yes, they do all sound slightly different.
If you're not that picky, then set up your cab on an absorptive surface such as carpet, set up all the baffling you're going to use (if any) such as acoustic cells, blankets, moving pads, etc., and check all your connections. Set up the 57 in front of one of the speakers 1 to 1 & 1/2 inches out from the center of the cone. Start with it pointing straight in at a perpendicular 90 degrees and give the tone a try. If you like, experiment with the angling of the mic at that position, listening to the tone after every tiny adjustment, to find your phase coloration preference. That's it.
Now mirror that same procedure with the 421 and either a) see which you like best, b) record a track with each of them separately, or c) record a track with both of them simultaneously. With the last option, you have to be careful to avoid phase cancellation issues, so you may need to listen to the recorded track and go back and experiment further with placement if you experience problems.
